[vsnet-alert 10913] Re: re FWD ATEL 1892

[Brian Skiff]

     From tests we've done using CMC14 r' with LONEOS camera CCD
images, the rms scatter on the Carlsberg data is something like
0.03 to 0.05 per star.  CMC is only a 7-inch refractor, so down
at mag 14 what do you expect?  Thus John's 0.06 rms scatter in the
transformation includes a fair contribution from the source data.
The CMC14 in VizieR shows the
number of photometric observations separate from the number used
for astrometry, and it's a good idea to select stars with more than
one observation obviously, and also to pick those where the quoted
errors on the photometry are less than say 0.04 mag or so.
Using five or six comp stars as John mentioned, is also beneficial.
The overall adjustment of CMC14 r' to Sloan r' is very good, 
within a couple percent at mag 15, where the two datasets overlap.
Since CMC14 shows the 2MASS JHK data, you can select comp stars to be 
similar in color to the target if there's an issue there.
Of course things with extreme colors and/or strong emission lines
are not in principle transformable at all to a
standard system based on plain-vanilla stars.

\Brian